Forty-two of the dead were calves. Most of the juvenile dolphins are washing up in Mississippi and Alabama, because dolphins typically give birth and raise calves along the shallow shores of those states.

The normal gestation period for the dolphins is one year, and mothers usually give birth in March and April, so scientists think the affected calves are either being aborted, stillborn, or born prematurely.

"That's one part of the investigation that we're going to be looking at very carefully," Mase said.

"We'll methodically score each animal that has come ashore to determine if, in fact, it was an aborted calf or an animal born alive."

Dolphin die-offs—which scientists call unusual mortality events—occur every few years. But this one stands out, because young dolphins appear to be hardest hit, marine biologist Moby Solangi said.

"Usually in a stranding, you have a mixture of animals—males, females, adults, calves—but this one is distortedly focused on neonates," said Solangi, director of the Institute for Marine Mammal Studies (IMMS) in Gulfport, Mississippi, which is helping to investigate the deaths.

Also unusual: Only dolphins appear to be affected so far. No mass deaths of turtles, fish, or birds have been reported for this die-off. (Also see "Why Are Birds Falling From the Sky?")

Known causes of dolphin die-offs include unusually cold waters, ocean biotoxins, and diseases.

NOAA's Mase said scientists are investigating all of these factors and are not ruling out a possible connection to the BP oil spill.

"It's something that we are including in our investigation," Mase said.

IMMS's Solangi agreed that the BP oil spill "is a factor that we need to consider."

"The oil spill lasted several months, and it covered tens of thousands of square miles and much of the habitat of these animals."

IMMS scientists are currently performing necropsies on the dead dolphins to try to determine causes of death. The process—including analyzing tissue samples for signs of diseases, viral infections, and toxins—could take several weeks or months, Solangi said.

While a link between last year's BP oil spill and this year's dolphin deaths is possible, it could be very difficult to prove, said Craig Matkin, a marine biologist at the North Gulf Oceanic Society in Alaska.

Matkin co-authored a study in 2008 that looked at the effects of the 1989 Exxon Valdez oil spill on killer whale populations in Alaska's Prince William Sound. (Read National Geographic magazine's 1990 coverage of the Exxon Valdez oil spill.)

"I'm not overly optimistic that they're going to be able to find a link," he said.

One reason is that—unlike other environmental toxins, such as the pesticide DDT—the hydrocarbon molecules in oil are quickly processed by the body and do not persist in tissues, Matkin explained.

Scientists also don't have a good idea of how the spill might have affected dolphins still in the womb.

Oil is thought to affect marine animals through inhalation or direct and indirect ingestion—for example, by eating tainted fish. But the calves now showing up dead may not have even been conceived before or during the worst weeks of the spill and thus were not exposed to the oil directly.

Dolphin Die-offs Largely Cold Cases

In his 2008 study, Matkin's team concluded that the Exxon Valdez spill affected Alaskan killer whale populations for decades after the event. After inhaling oil vapors or eating oil-coated seals, for example, the whales experienced everything from "mild irritation" to instant death, the study days.

It's unknown how the 1989 spill affected calves. Killer whales tend to give birth in deep water, so dead calves are much less likely to wash ashore.